The present invention relates to a motor vehicle suspension system, with hydropneumatic spring units which each consist essentially of a hydraulic displacement unit arranged between a sprung mass, e.g. the vehicle body, and an unsprung mass, e.g. a vehicle axle, and, more particularly to a suspension system with at least one especially pneumatic spring accumulator connected hydraulically to the displacement unit and a compensating arrangement for suppressing critical relative vibrations between the sprung and the unsprung masses.
A suspension system is shown in German Auslegeschrift No. 1,430,836 wherein arranged between the body of the vehicle and its axles or wheels, are respective hydropneumatic spring units which each have a displacement unit configured as a piston/cylinder unit and a hydropneumatic accumulator connected thereto via a throttle and possessing a main gas cushion acting as a spring. A further smaller hydropneumatic accumulator is connected in a non-throttled manner to the displacement unit and possesses an auxiliary gas cushion acting as a spring. In this arrangement, low-frequency vibrations between the sprung and the unsprung mass are damped by the throttle between the displacement unit and the first-mentioned hydropneumatic accumulator. In contrast, vibrations of higher frequency are damped only extremely weakly because the other hydropneumatic accumulator is connected in a non-throttled manner to the displacement unit. In order nevertheless to make it possible effectively to suppress vibrations even of higher frequency, the unsprung mass is coupled to a compensating arrangement rated for these vibrations of higher frequency.
This known arrangement allows for the fact that, with a view to a high degree of comfort, it is basically desirable, in all suspension systems for motor vehicles, to keep the damping of the system, that is to say the friction occurring in the system, as low as possible. However, this is not directly possible because resonant vibrations can occur, specifically, where motor vehicles are concerned, especially a relatively low-frequency body resonance and an axle resonance of comparatively higher frequency. Typical values for the body resonance are around 1 Hz, while the axle resonance occurs at approximately 13 Hz, i.e. a frequency approximately 10 times that of the body resonance.
An object of the present invention is to provide a suspension system which can work with increased damping in critical resonant ranges and which is characterized at the same time by an especially simple construction.
This object has been achieved in accordance with the invention by a suspension system in which parts of the hydraulic medium can be excited as a compensating mass, in relation to the sprung mass, to vibrations whose natural frequency coincides at least approximately with the frequency of the critical relative vibrations.
With a view to an especially effective suppression of critical relative vibrations, the system is so configured that relative movements between the sprung and the unsprung mass lead to comparatively high speeds of movement of the hydraulic medium.
The present invention is based on the general concept of utilizing the moved mass of hydraulic medium as a compensating mass at critical frequencies of the suspension system and thereby achieving a frequency-selective dynamic damping. At the same time, a suitable hydraulic transmission ratio can ensure that a relatively small quantity of hydraulic medium is moved at high speed in order to achieve a good compensating effect, despite the low mass of moved hydraulic medium.
According to a first advantageous embodiment of the present invention, the displacement unit is connected to the sprung mass via an elastic bearing part and to the associated spring accumulator via a hydraulic line, while the spring constant of the bearing part, the mass of that part of the displacement unit or of the spring unit mounted on the bearing part, the cross-section of the hydraulic line and its length, the effective cross-section of the displacement unit and the density of the hydraulic medium are so calculated or coordinated with one another that the resonant frequency of the spring unit is near a critical frequency of the suspension system, for example near the frequency of the axle resonance of a vehicle.
It is a feature of a further preferred embodiment of the present invention that the displacement unit is connected to a first spring accumulator, and the latter is connected to a second spring accumulator via a hydraulic channel. The length and cross-section of the channel are so coordinated with the density of the hydraulic medium and with the characteristics of the spring accumulators that the natural frequency of vibrations of the fluid mass in the channel coincides at least approximately with one of the critical frequencies of the suspension system, again for example the frequency of the axle resonance of a vehicle.
The latter preferred embodiment is characterized by an especially high compensating effect. Moreover, a high freedom of construction is guaranteed in the coordination of the suspension system. It is to be stressed particularly that the elasticity of bearing parts to be arranged, if appropriate, between the sprung mass and the displacement or spring unit can be set largely as desired, so that, for example in motor vehicles, the best possible wheel guidance can be guaranteed.